NO20231395A1 - BOP-stack, system and method - Google Patents

Description

Title: BOP-stack, system and method

Field of invention

The invention relates to a BOP-stack, a system and a method for drilling when using a floating single derrick drilling unit or a floating dual derrick drilling unit.

Background

Conventional drilling offshore requires a marine riser to be run to act as a conduit for the drilling fluids back to the rig after the BOP has been run.

The inventor has realized that the necessity of a riser introduces various problems and therefore the inventor has attempted to devise a system that permits riserless drilling, i.e. drilling wells without the use of a marine riser.

The motivation of the inventor to solve the riserless drilling challenge originates from an efficiency standpoint. When using a dual derrick rig or drillship, both derricks can be utilized in a very efficient way during top hole drilling, allowing both derricks to be used extensively for all operations. However, as soon as the BOP is run, all operations are limited to only one derrick, since all operations must be run through the marine riser leading down to the well.

By removing the marine riser (Riserless Drilling), as is enabled by the invention discussed below, then both derricks can continue to be used extensively also after the BOP has been run. A significant amount of time and cost can be saved if this is achieved.

Summary of the invention

Viewed from a first aspect the invention provides a BOP stack for riserless drilling comprising a frame structure for mounting equipment, a BOP high pressure section for handling well incidents, a funnel guide with debris catcher at the top of the BOP stack, and components positioned below the funnel guide and above the BOP high pressure section comprising a sealing unit with a sealing element for stopping the flow in a return mud annulus if needed, a return mud manifold for guiding return mud from a return mud annulus to an interface with return mud hoses fluidly connecting the BOP stack to a topside mud handling system, a flange for connecting the wellbore components to the BOP stack, and interfaces for return mud hoses and power and control lines. The BOP stack may comprise a control unit for controlling the pump module, sensors and valves The BOP stack comprises a subsea pump module for pumping return mud to a topside mud handling system, wherein the subsea pump module is connected to a suction module fluidly connected to the return mud annulus below the sealing unit. By use of the BOP stack of the first aspect, increased efficiency associated with the use of the riserless dual derrick drilling system can advantageously give a time reduction of more than 10% on wells drilled in shallow waters and may contribute to time savings above 20% for wells drilled in deeper waters (compared with a dual derrick semi-sub rig without riserless drilling capabilities, even more if compared with a single derrick rig). The components may for example be wellbore components, e.g. components that are used for and/or specifically configured for wellbore operations at the BOP stack as distinct from components that are not used for wellbore operations.

For the time saved on each well, the corresponding running costs are also eliminated, resulting in a significant cost benefit. For every day that is saved due to increased drilling efficiency, emissions will be cut by a similar percentage. Also, the operator will experience an increased operational window since Lower Marine Riser Package (LMRP) disconnect requirements no longer are applicable.

Furthermore, wellhead fatigue related challenges are eliminated.

By removing the riser, shallow water Dynamic Positioning (DP) is made possible. Thus, the invention may include methods comprising drilling in shallow water using DP. The BOP stack of the first aspect and/or the system of the second aspect may be included in a system for drilling in shallow water using DP. In prior art drilling in shallow water using DP has been impossible because an offset from the center position would very quickly cause the angle between the BOP stack and the riser to exceed the breaking point of the riser. The following advantages can arise from using riserless DP drilling in shallow waters, such as by use of the preferred embodiments described herein:

• Removing the need for anchor handling and associated cost.

• Improving the flexibility of the rig (easier to temporarily leave a well, etc.).

• Optimizing rig movement due to optimal rig heading.

• Improving cargo handling from supply vessels.

• Reduce heavy lifts on the rig by not having to run the marine riser, thus improving HSEQ.

• Reduce the active volume due to considerably smaller return riser.

The wellbore components of the BOP stack may comprise a mud level cylinder just below the funnel guide for balancing the mud level during operation.

The BOP stack may comprise a mud level system comprising at least one mud sensor for determining the mud level in the mud level cylinder and funnel guide, wherein the mud level system is configured to control the pumping of the pumping module based on signals from the at least one mud sensor to keep the mud level within a maximum level and a minimum level.

The return mud manifold may comprise a bypass pipe bypassing the subsea pump module and allowing return mud to flow from the return mud annulus to the interfaces of the return mud hoses without entering the pump module, wherein a bypass valve is mounted on the bypass pipe.

The subsea pump module may be mounted on a mud return main pipe having a first main pipe valve between the suction module and the pump module and a second main pipe valve between the pump module and the interface with the mud return hose.

The return mud manifold may comprise an extraction pipe connected between the upstream side of the pump module and the funnel for extracting polluted fluids from the funnel, wherein the funnel pipe is provided with a funnel valve.

The BOP stack may comprise a riser spool piece with a shuttle tool groove to mate with a shuttle tool, wherein the riser spool piece is positioned below a wear bushing positioned below the funnel guide.

The BOP stack may be designed for redundancy by comprising at least two pump modules, the mud manifold may comprise at least two mud return main pipes each having respective first main pipe valve, the pump module and second main pipe valve mounted in that order from the mud return annulus, at least two mud fill pipes for fill-up and priming of pumps with respective mud fill valves coupled to respective mud return main pipes between respective pump modules and first main pipe valves. The bypass pipe may be fluidly connected to at least two mud return main pipes above the second main pipe valve and the funnel pipe may be fluidly connected to at least two mud fill pipes.

In a second aspect the invention provides a system for riserless drilling comprising a BOP stack for riserless drilling according to the first aspect, optionally including any of the other features discussed above. The system may include a floating drilling unit comprising at least one derrick, a return mud hose, a mud fill hose, kill and choke lines, and power and control lines mounted on a reel positioned on the floating drilling unit. The system may comprise an Automatic Heave Compensation (AHC) system for all lines and hoses.

The system may comprise an Emergency Quick Disconnect (EQD) system for disconnecting the return mud hoses and kill and choke lines in case of drift off. The EQD system may comprise disconnectable hose connection units for mud hoses and kill and choke lines, an extension line connected to the disconnectable part of the mud hoses and kill and choke lines and the AHC system, wherein the extension line can be paid out as the floating drilling unit drifts off and the extension line i s long enough to lay the mud hose and kill and choke lines on the ocean floor before being cut or released. The QED system may comprise a reel paying out power and control lines.

The system may comprise a Dynamic Positioning (DP) system for the floating drilling unit.

An anchoring system may be included in the system for keeping the floating drilling unit in position.

The floating drilling unit may be a dual derrick drilling unit comprising a first derrick and a second derrick, wherein the first and second derrick is arranged for drilling interchangeably in the same wellhead.

In a third aspect the invention provides a method for riserless drilling using the system for riserless drilling, advantageously wherein the floating drilling unit is a floating dual derrick drilling unit. The method may include using a system with any of the features discussed above, including features discussed in context of the first aspect. The method may comprise the steps of:

a. executing a first drilling operation with the first derrick while preparing a second drilling operation with the second derrick,

b. executing a second drilling operation with the second derrick while preparing a third drilling operation with the first derrick.

In another aspect the invention provides a method for riserless drilling using the system for riserless drilling comprising the QED system and DP system described above. The method may comprise the steps of:

a. executing a drilling operation,

b. if the floating drilling unit exceeds a safe drilling region, activating the Emergency Quick Disconnect (EQD) system for disconnecting the return mud hoses and kill and choke lines by disconnecting the disconnectable hose connection units wherein the extension line is paid out as the floating drilling unit drifts off and lays the mud hose and kill and choke lines on the ocean floor before the extension line is cut or released.

Viewed from a further aspect, which is not currently independently claimed, the invention provides an environmental protection riser for preventing release of polluted fluid when drilling with Oil Based Mud (OBM). This environmental protection riser may be used together with any of the other aspects discussed above, including in some implementations in combination with the optional features of those aspects. The environmental protection riser may comprise a main tube comprising a plurality of tubular sections for enveloping the drill st ring from a wellhead to a floating drilling unit and a connector pin in the lower end of the tubular for mating with a connector housing on a BOP stack. The Environmental protection riser further comprises a weak link break-off joint above the connector pin, and a flexible joint above the weak link break off joint.

In still further aspects the invention provides a system for drilling interchangeably with and without the environmental protection riser of the above aspect. The system may include features of the system of the second aspect and/or it may comprise a BOP stack having any of the features set out above in relation to the first aspect. The system may comprise a floating drilling unit comprising at least one derrick, a BOP stack for drilling interchangeably with and without environmental protection riser. The system preferably comprises a frame structure for mounting equipment, a funnel guide with debris catcher at the top of the BOP stack, a connector housing below the funnel guide for receiving the connector pin of the environmental protection riser and wellbore components positioned below the mud level cylinder comprising a sealing unit with a sealing element for stopping the flow in a return mud annulus if needed, a return mud manifold for guiding return mud from a return mud annulus to an interface with return mud hoses fluidly connecting the BOP stack to a topside mud handling system, and a flange for connecting the subsea apparatus to a lower stack. The BOP stack may comprise interfaces for return mud hoses and power and control lines, and a subsea pump module for pumping return mud to a topside mud handling system, wherein the subsea pump module is connected to a suction module fluidly connected to the return mud annulus below the sealing unit.

In another aspect the invention provides a method for drilling interchangeably with and without environmental protection riser using the system for drilling interchangeably with and without environmental protection riser described above. This method may include method steps described above in connection with the other method aspects. The method may comprise the steps of:

a. when drilling with Water Based Mud (WBM), drilling in a riserless mode,

b. when drilling with Oil Based Mud (OBM) is required, retracting the drill string, and

c. lowering the connector pin of the environmental protection riser into the connector housing and proceed with drilling using OBM with the environmental protection riser in place.

Brief description of the drawings

To facilitate the understanding of the invention, there are attached figures illustrating different example embodiments of the invention where like numbers in different figures refer to the same features.

Fig. 1 shows an embodiment of a BOP stack

Fig. 2 show another embodiment of a BOP stack with a view of the wellbore components.

Fig. 3 shows an embodiment of a BOP stack mounted on a wellhead

Fig. 4 shows an embodiment of a dual derrick drilling unit

Fig. 5 shows an embodiment of an environmental protection riser

Fig. 6 shows an embodiment of a floating drilling unit with an Emergency Quick Disconnect (EQD) system.

Fig. 7 shows a schematic view of a variant of the mud manifold with increased redundancy.

Detailed description

Certain example embodiments will now be described in further detail by way of example only.

As set out herein, a BOP stack 1 for riserless drilling and a system for riserless drilling using the BOP stack may be combined with a floating drilling unit 22. The floating drilling unit 22 comprises at least one derrick and may be a floating single derrick drilling unit or a floating dual derrick drilling unit. Also, a method for using the system for riserless drilling is described. The system for riserless drilling and the BOP stack is first described in context of a design for Water Based Mud (WBM) only. In a separate section it will be described how the system and BOP stack can be modified to allow for drilling with Oil Based Mud (OBM).

Fig. 1, 2 and 3 shows the BOP-stack 1 for riserless drilling comprising a standard BOP stack suitable for the well at hand and a modified riserless version of a Lower Marine Riser Package (LMRP). The BOP stack and LMRP work together. The riserless version of the LMRP is smaller than the standard LMRP and therefor easier to integrate into the BOP stack. The integration of the BOP stack and the riserless version of the LMRP gives an efficiency advantage and we call this combined unit a ‘BOP stack for riserless drilling’. For convenience we will often use only BOP stack 1. The BOP stack comprises a BOP high pressure section 27 for handling well incidents. The BOP high pressure section 27 will be adapted to the well in question. This part of the BOP stack is well known in the industry and will not be described in further detail.

The BOP stack 1 for riserless drilling comprises a frame structure 2 for mounting equipment and a funnel guide 3 with a debris catcher 4 functioning as a protective plate positioned at the top of the BOP stack as seen in fig. 2. Preferably, a mud level cylinder 11, described in detail later, is positioned below the funnel guide 3. The BOP stack comprises wellbore components positioned below the funnel guide 3 and also below the mud level cylinder 11, if present. The wellbore components comprise a sealing unit 20 with a sealing element for stopping the flow in a return mud annulus 5 if needed, and a mud manifold 16 for guiding return mud from a return mud annulus 5 to an interface 28 with return mud hoses 6 fluidly connecting the BOP stack 1 to a topside mud handling system 12. The mud manifold 16 comprises the piping needed to run the pump module 10 and direct the mud flows. Obviously, the mud manifold 16 can have a large number of embodiments. Fig 1 and 2 only show the most essential features, while fig. 7 shows an embodiment designed for redundancy to avoid single point failures. The wellbore components also comprise a flange 7 for connecting the riserless version of the LMRP to a lower part of the BOP stack 1. The BOP stack further comprises interfaces 28 for return mud hoses 6, kill and choke lines 8 and power and control lines 9.

The BOP stack comprises a subsea pump module 10 for pumping return mud to a topside mud handling system 12, wherein the subsea pump module 10 is connected to a suction module 41 fluidly connected to the return mud annulus 5 below the sealing unit 20. Fig. 1 and 2 shows an embodiment of the suction module 41, which comprises a tubular integrated into the BOP stack between the sealing unit and the flange 7 and having two outlets. Fig. 7 shows an embodiment of the suction module with three outlets. The return mud will be pumped back to the top side mud handling system 12 by the pump module 10 with top of the BOP open to sea, and where the return mud flow is regulated based on the level of mud in the funnel guide 3 and/or mud level cylinder 11. Obviously, the BOP stack 1 has a controller unit for controlling the pump module 10, sensors, valves or other equipment to run the BOP stack, but this is well known in the industry and will not be described in this text.

As previously mentioned, the wellbore components preferably comprise a mud level cylinder 11 just below the funnel guide 3. The purpose of the mud level cylinder is to provide a larger range of levels within which the mud can be balanced during operation. The mud level must be high enough to avoid the pumps sucking in air and low enough to avoid overflow of the funnel guide. In an embodiment of the BOP stack this balance can be provided by a mud level system comprising at least one mud sensor 18 for determining the mud level in the mud level cylinder 11 and/or funnel guide 3. The mud level system is configured to control the pumping of the pumping module 10 based on signals from the at least one mud sensor 18 to keep the mud level within a maximum level and a minimum level. The mud level system may also comprise mud fill pipe 51 fluidly connecting the pump module 10 with mud fill hoses 49 connected to the topside mud handling system 12. The balancing operation will be performed with the BOP-stack open to sea where the subsea pump module 10 is used to equalize the pressure towards the sea and keep a mud/water interface in the funnel at predefined level between the mentioned minimum and maximum mud level inside the mud level cylinder 11.

During tripping operations, the subsea pump module 10 will be regulated to keep the same mud level in the BOP stack 1. The mud fill hoses 49 will supply mud from the top side mud handling system 12 to keep the mud level balanced mud level cylinder and/or in the funnel, i.e. during tripping out. In an embodiment the mud handling system 12 comprises trip tanks connected to the mud fill hoses 51. The purpose of the mud fill hose 49 and mud fill pipe 51 is also to always ensure a minimum of mud circulation for the subsea pumps.

In an embodiment, shown in fig. 7, the mud manifold 16 comprises a bypass pipe 29 bypassing the subsea pump module 10 allowing return mud to flow from the return mud annulus 5 to the interfaces 28 of the return mud hoses 6 without entering the pump module 10. Preferably, a bypass valve 48 is mounted on the bypass pipe 29 to avoid backflow when the bypass pipe is not in use. The bypass pipe 29 can be used when the pump module 10 is defect. Extra pressure can in such instances be provided through the drill string 14 pushing the return mud through the bypass pipe 29 past the pump module 10. This will only work if the sealing unit 20 is closed, otherwise the extra pressure would cause return mud to partly flow out of the funnel guide 3. If the drilling operation demands milling through casings or drilling through plugs causing debris that is likely to damage the pump modules 10 the bypass pipe 29 can be activated when the mud containing the harmful debris passes the BOP stack 1. However, drilling with a closed sealing element causes a lot of wear on the sealing element and should be kept at a minimum.

In an embodiment, shown in fig. 7 the pump module 10 is mounted on a mud return main pipe 45 having a first main pipe valve 46 between the suction module 41 and the pump module 10 and a second main pipe valve 47 between the pump module 10 and the interface 28 with the mud return hose 6. The first and second main pipe valves 46, 47 will allow the operator to isolate the pump module 10 if blocked or leaking or to facilitate use of the bypass pipe 29 when needed. Preferably, the return mud manifold 16 comprises manifold exit valves 54 close to the interfaces 28 of the mud return main pipes 45 to prevent outflow.

In a preferred embodiment the return mud manifold 16 comprises a funnel extraction pipe 42 connected between the upstream side of the pump module 10 and the funnel 3 for extracting polluted fluids from the funnel. This might occur when extracting the drill string 14 from the wellhead 13. The funnel extraction pipe 42 is provided with a funnel valve 43.

In a preferred embodiment shown in fig. 7 the funnel guide 3 will be equipped with a debris outlet pocket 40 to recover sponge balls and larger debris from the wellbore. This is to reduce the wear and tear on the pump modules 10 and reduce the possibility for plugging of the mud manifold 41.

In order to transport the BOP stack 1 to the wellhead 13 the wellbore components of the BOP stack 1 may comprise a riser spool piece 17 with a shuttle tool groove 44 to mate with a shuttle tool, wherein the riser spool piece 17 is positioned below a wear bushing 15 positioned below the funnel guide 3.

During well control situations, an annular preventer mounted on the BOP stack below the mentioned wellbore components in the mentioned BOP high pressure section 27 is to be used for sealing, and flexible Kill & Choke lines are to be used for circulation. The wellbore components, suction module 41 and mud manifold 16 is not planned to be used during these situations and need not be qualified for the pressure associated with well control situations (except for the routing of the Kill & Choke lines). The valves and equipment needed to handle well control incidents is well known in the industry and will not be described in any further detail in this text.

Redundancy

In a preferred embodiment the BOP stack 1 for riserless drilling is designed for redundancy to avoid single point failures. Basically, the BOP stack 1 is provided with at least two of all the important features. The BOP stack comprises at least two pump modules 10 and to achieve a reasonable degree of redundancy each pump module 10 must have the capacity to maintain normal operation. A failure of an active component or a valve, hose or pipe connected to one of the pump modules 10 must not jeopardize the operation of the other pump module 10. Furthermore, it must be possible to operate with both systems in parallel when both systems are operable, and control of required subsea valves must be possible from both control systems.

These effects are achieved by the features described in this section with references to fig. 7. The mud manifold 41 is made redundant by comprising at least two mud return main pipes 45. Each return main pipe 45 has the respective first main pipe valve 46, pump module 10 and second main pipe valve 47 mounted in consecutive order from the suction module 41 connected to the mud return annulus 5. This will facilitate isolation of a damaged pump module.

In an embodiment the redundant mud manifold 16 comprises at least two mud fill pipes 51 for fill-up and priming of pumps with respective mud fill valves 52 coupled to respective mud return main pipes 45 between respective pump modules 10 and first main pipe valves 46. Conceivably, the mud manifold 16 can be provided without mud fill pipes 51, but this would reduce the efficiency of the pumping function. The bypass pipe 29 is fluidly connected to at least two mud return main pipes 45 above the second main pipe valve 47 and the funnel pipe 43 is fluidly connected to at least two pump modules 10, preferably via the mud fill pipes 51.

System for riserless drilling

A system for riserless drilling is described below. The system for riserless drilling, hereinafter called ‘the system’, comprises a BOP stack 1 for riserless drilling as described above and a floating drilling unit 24. The floating drilling unit 24 may be a single derrick drilling unit (not shown) or dual derrick drilling unit 24 comprising a first derrick 25 and a second derrick 26, wherein the first and second derrick 25, 26 both are able to drill interchangeably in the same wellhead 13. The system further comprises a return mud hose 6, a mud fill hose 27, and kill and choke lines 8. The system further comprises power and control lines 9 mounted on a reel 30 positioned on the floating drilling unit 22. The function of the reel will be explained later. Preferably the system further comprises an Automatic Heave Compensation (AHC) system 31 for all lines and hoses 6, 8, 9. The lines and hoses may each have an AHC system, or some or all the lines and hoses may be bundled together.

In a preferred embodiment shown in fig. 4 and 6, the kill and choke lines, the mud return hoses 6 and mud fill hoses 27 to BOP stack is designed with dedicated length for the water dept. One set of hoses bundled together forward of a moon pool and one set of hoses bundles together aft. Preferably a kill line in one of the bundles and a choke line in the other, together with a mud fill hose 49 and return mud hose 6 in each. The three different hoses in each bundle (return mud hose 6, mud fill hose 49 and kill/choke 8) will be terminated to a common connection plate at each termination. The kill/choke line carry the load of the segments and will be slightly shorter than the other hoses.

In an embodiment shown in fig. 6 the system comprises an Emergency Quick Disconnect (EQD) system 32 for disconnecting the return mud hoses 6 and kill and choke lines 8 in case of drift off. An efficient and easy to operate QED system will allow a floating drilling unit 22 held in place by a Dynamic Positioning (DP) system to operate without risk of breaking the BOP stack 1 and/or the drill string 14. The EQD system comprises disconnectable hose connection units 50 for return mud hoses 6 and kill and choke lines 8. The hose connection units 50 connects and disconnects the mud return hoses 6 and the kill and choke lines 8 and is connected to the AHC system 31 via an extension line 53 such that the lower part of the mud hoses 6 and kill and choke lines 8 remains fastened to the AHC system 31 via the extension line 53. The extension line can be paid out as the floating dual derrick drilling unit drifts off. The extension line 53 is long enough to lay the return mud hose 6 and kill and choke lines 8 on the ocean floor in a controlled manner before being cut or released. The mud fill hose(s) 49 may also be bundled together with the respective return mud hose 6 and kill and choke lines 9 to make two bundles of hoses 6, 8, 49 as seen in fig. 6. The bundles hoses are connected to the same hose connection unit 50. Preferably the EQD is activated by an activation signal to the Active Heave Compensation system 31 to ensure a controlled spool out of the winch with dropping of the bundled hoses down to the seabed.

In a preferred embodiment the EQD system 32 comprises at least one reel 30 for paying out power and control lines 9. The EQD system comprises extensions of the power and control lines rolled up on the reel 30, thus enabling control of the BOP stack 1 a period of time after an incident involving drift off. When control of the position of the floating drilling unit is reestablished, the lines and hoses can be picked up from the ocean floor and reconnected.

In an embodiment of the system for riserless drilling the floating drilling unit 22 is a floating single derrick drilling unit. In an alternative and preferred embodiment, the floating drilling unit 22 is a dual derrick drilling unit 24 as shown in fig. 4 and 6. The system may comprise a Dynamic Positioning (DP) system using thrusters 33 to position the floating drilling unit 22. Dynamic Positioning systems are well known in the industry and will not be further described in this text.

As an alternative to the DP system the system for riserless drilling may comprise an anchoring system. In some embodiments the floating drilling unit 22 is a floating dual derrick drilling unit 24 comprising means for movement between a first position having the first derrick 25 straight above the wellhead 13 and a second position having the second derrick 26 straight above the wellhead 13. Obviously, the DP system is by far much faster to operate than the anchoring system, especially in the embodiments where the floating drilling unit 22 is a floating dual derrick drilling unit 24 that repeatedly moves between the two mentioned first and second positions.

As for the BOP stack the system for riserless drilling may advantageously be made redundant to prevent single point failures. In particular, the system can be provided with two return mud hoses 6 and preferably also two mud fill hoses 49.

Drilling with Oil Based Mud (OBM)

It is not always possible to guarantee that all sections of a well to be drilled will be drilled successfully with WBM. It is therefore an advantage to have a contingency solution where it is possible to drill a section with OBM without having to wait for prolonged amount of time before being ready to resume drilling. Even though the riserless drilling system in itself works fine, having the ability to quickly switch to an OBM qualified system may be important to ensure that the non-productive time (due to subsurface challenges) associated with the riserless drilling system is kept to a minimum.

An environment protection riser 34, shown in fig. 5, may be introduced as a mitigation for the possible challenges associated with OBM. Overall, the environmental protection riser 34 and a related system for drilling with OBM may be the same riser as the conventional riser, but without the kill/choke/boost and conduit lines attached. This will reduce the overall running time as it’s not necessary to orientate the joint. The environment protection riser 34 will be filled with seawater and the riserless dual gradient philosophy will be used combining the pressure from the water column above the wellhead 13 and the mud pressure below the wellhead 13. As with the system for riserless drilling the mud water interface 21 will be at or below the funnel guide 3 as indicated in fig. 2. In an event of an emergency disconnect, the discharge to sea will only be a limited amount of polluted water from the environmental protection riser 34. In most cases the funnel pipe 42 will be able to suck out polluted water from the lower part of the environmental protection riser 34 before being exposed to the open sea.

In an embodiment the system for riserless drilling is expanded to include an environmental protection riser 34 for drilling intermediate sections with OBM and preventing release of polluted fluid when drilling with OBM. As shown in fig. 5 the environmental protection riser 34 comprises a main tube 35 for enveloping the drill string 12, a connector pin 36 in the lower end of the tubular for mating with the connector housing 39 and a flexible joint 37 above the connector pin 36.

Furthermore, the environmental protection riser comprises a weak link break-off joint 38 above the connector pin 36 and below the flexible joint 37. For the connector pin to connect to the BOP stack 1 a connector housing 39 for receiving the connector pin 36 is provided below the funnel guide 3.

If the well design is set for OBM in an intermediate section of the formation and the floating drilling unit 22 is a floating dual derrick drilling unit 24 , the environment protection riser 34 can be run offline in the first derrick 25 while the second derrick 26 is running and cementing previous casing. The only online activity would be the pulling the environmental protection riser 34 prior to drill next WBM section.

Operational sequences

The main purpose of the system for riserless drilling is to be more efficient than standard drilling operations with a marine riser. To achieve this, it is essential that most, if not all operations are run in alternating derricks meaning that the floating drilling unit 22 should be a dual derrick drilling unit 24 comprising a first derrick 25 and a second derrick 26 as indicated in fig. 4. The table below shows an example of how the operations may be performed drilling with a dual derrick drilling unit 24 in a typical shallow water well (100 – 150 m water depth).

MU – Make Up

BHA - Bottom Hole Assembly

LD – Lay Dowm

POOS - Pull Out Of Stack/Sea

RT – Reservoar Tool

WL – Wireline

M/U – Make Up

R/U – Rig Up

RIS – Rin in Sea

CAST/CBL Circumferential Accustic Scanning Tool/Cement Bond Log CBP/MRT – Lower Completion Running Tool

THRT - Tubing Hanger Running Tool

In a typical well, as we can see from the table, the time savings are equivalent to approximately 50, 4 hrs. or 2.1 days. Out of 20 days, this gives a time reduction of 10.5%, including the top hole sections. Excluding the top hole sections, the time reduction is equivalent to approximately 12.5% (2.1 out of 17 days). The time savings are dependent on factors like water depth (larger water depth means more operations can be performed offline), number of sections, length of sections, well type (exploration, production, MLT, producer, water injector, etc.) and complexity of completion. The time saved on anchor handling is not calculated.

The system for riserless drilling has a number of positive effects: Anchor handling is not necessary because riserless drilling allows Dynamic Positioning systems to be used for positioning the floating drilling unit. Deeper water will add to the potential savings when using a dual derrick system. The flexibility increases because moving the floating drilling unit 22 to a different location do not involve anchor handling. Handling of supply vessels will be made easier because the DP system allows the floating drilling unit 22 to adjust the heading of the floating drilling unit 22 to suit the weather conditions.

Most of the time saved is due to the combination of having access to two derricks and not having a riser. This will to a large degree enable preparing one operation with one derrick while executing another operation with the other derrick as indicated in fig. 4. A method for riserless dual derrick drilling using the system for riserless drilling with a floating dual derrick drilling unit 24 described previously comprises the steps of executing a first drilling operation with the first derrick 25 while preparing a second drilling operation with the second derrick 26, and executing a second drilling operation with the second derrick 26 while preparing a third operation with the first derrick 25.

When using a system for riserless drilling comprising a floating drilling unit 22 and a Dynamic Position (DP) system the method for riserless comprises the steps of executing a drilling operation, and if the floating drilling unit 22 drifts outside a region causing the drill string 14 to exceed a maximum angle, the Emergency Quick Disconnect (EQD) system 32 is activated for disconnecting the return mud hoses 6 and kill and choke lines 8 by disconnecting the disconnectable hose connection units 50 wherein the extension line 53 is paid out as the floating drilling unit 22 drifts off and lays the mud hose 6 and kill and choke lines 8 on the ocean floor in a controlled manner before the extension line 53 is cut or released. In an embodiment of the method the power supply and control lines 9 are paid out by the reels 30 they are rolled up on as the floating drilling unit 22 drifts off at least until the return mud hose 6 and kill and choke lines are laid on the ocean floor before power supply and control lines 9 are cut off or disconnected.

References

1 BOP stack for riserless drilling

2 Frame structure

3 A funnel guide

4 Debris catcher

5 Return mud annulus

6 Return mud hoses

7 Flange for connecting wellbore components to BOP stack

8 Kill and Choke lines

9 Power supply & control lines)

10 Pump module

11 Mud level cylinder

12 Topside mud handling system

13 Wellhead

14 Drill string

15 Wear bushing

16 Return mud manifold

17 Riser spool piece with shuttle tool groove

18 Mud level sensors

20 Sealing unit with seal element

21 Mud / water interface

22 Floating drilling unit

24 Floating dual derrick drilling unit

25 First derrick

26 Second derrick

27 Blow-Out Preventer (BOP) high pressure section

28 Interfaces for hoses, lines and power

29 Bypass pipe

30 Reel for power and control lines

31 Active Heave Compensation (AHC) winch

32 Emergency Quick Disconnect (EQD) system

33 Rig thrusters (in use when operating on Dynamic Positioning (DP)) 34 Environmental protection riser

35 Main tube of environmental protection riser

36 Connector pin

37 Flexible joint

38 Weak link break off joint 40

39 Connector housing

40 Debris outlet pocket

41 Suction module

42 Funnel extraction pipe

43 Funnel valve

44 Shuttle tool groove

45 Mud return main pipe

46 First main pipe valve

47 Second main pipe valve

48 Bypass valve

49 Mud fill hose

50 Disconnectable hose connection units for the return mud hose and kill and choke lines

51 Mud fill pipes

52 Mud fill valves

53 Extension line

54 Manifold exit valve

Claims (15)

1. BOP stack (1) for riserless drilling comprising:

a frame structure (2) for mounting equipment,

a BOP high pressure section (27) for handling well incidents, a funnel guide (3) with debris catcher (4) at the top of the BOP stack, components positioned below the funnel guide (3) and above the BOP high pressure section (54) comprising:

a sealing unit (20) with a sealing element for stopping the flow in a return mud annulus (5) if needed,

a return mud manifold (16) for guiding return mud from a return mud annulus (5) to an interface (28) with return mud hoses (6) fluidly connecting the BOP stack (1) to a topside mud handling system (12?), and

flange (7) for connecting the wellbore components to the BOP stack (27),

interfaces (28) for return mud hoses (6) and power and control lines (9),

characterized by

a subsea pump module (10) for pumping return mud to a topside mud handling system (12), wherein the subsea pump module (10) is connected to a suction module (41) fluidly connected to the return mud annulus (5) below the sealing unit (20).

2. BOP stack (1) for riserless drilling according to claim 1, wherein the wellbore components comprise a mud level cylinder (11) just below the funnel guide (3) for balancing the mud level during operation.

3. BOP stack (1) for riserless drilling according to claim 2, wherein the BOP stack (1) comprises a mud level system comprising at least one mud sensor (18) for determining the mud level in the mud level cylinder (11) and funnel guide (3), wherein the mud level system is configured to control the pumping of the pumping module (10) based on signals from the at least one mud sensor (18) to keep the mud level within a maximum level and a minimum level.

4. BOP stack (1) for riserless drilling according to any of the previous claims, wherein the return mud manifold (41) comprises a bypass pipe (29) bypassing the subsea pump module (10) allowing return mud to flow from the return mud annulus (5) to the interfaces of the return mud hoses (6) without entering the pump module (10), wherein a bypass valve (48) is mounted on the bypass pipe (29).

5. BOP stack (1) for riserless drilling according to any of the previous claims, wherein the subsea pump module is mounted on a mud return main pipe (45) having a first main pipe valve (46) between the suction module (41) and the pump module (10) and a second main pipe valve (47) between the pump module (10) and the interface (28) with the mud return hose (6).

6. BOP stack (1) for riserless drilling according to any of the previous claims, wherein the return mud manifold (16) comprises an extraction pipe (42) connected between the upstream side of the pump module (10) and the funnel (3) for extracting polluted fluids from the funnel, wherein the funnel pipe (42) is provided with a funnel valve (43).

7. BOP stack (1) for riserless drilling according to any of the previous claims, wherein the BOP stack (1) comprises a riser spool piece (17) with a shuttle tool groove (44) to mate with a shuttle tool, wherein the riser spool piece (17) is positioned below a wear bushing (15) positioned below the funnel guide (3).

8. BOP stack (1) for riserless drilling according to any of the previous claims, wherein the BOP stack (1) for riserless drilling (1) is designed for redundancy by comprising:

at least two pump modules (10),

the mud manifold (41) comprising:

at least two mud return main pipes (45) each having respective first main pipe valve (46), the pump module and second main pipe valve (47) mounted in order from the mud return annulus (5),

at least two mud fill pipes (51) for fill-up and priming of pumps with respective mud fill valves (52) coupled to respective mud return main pipes (45) between respective pump modules (10) and first main pipe valves (46), wherein the bypass pipe (29) is fluidly connected to at least two mud return main pipes (45) above the second main pipe valve (47) and the funnel pipe (43) is fluidly connected to at least two mud fill pipes (51).

9. System for riserless drilling comprising:

a BOP stack (1) for riserless drilling according to any of the previous claims,

a floating drilling unit (22) comprising at least one derrick, a return mud hose (6),

a mud fill hose (27),

kill and choke lines (8),

power and control lines (9) mounted on a reel (30) positioned on the floating dual derrick unit (24),

an Automatic Heave Compensation (AHC) system (31) for all lines and hoses (6, 8, 9).

10. System for riserless drilling according to claim 9, wherein the system comprises an Emergency Quick Disconnect (EQD) system (32) for disconnecting the return mud hoses (6) and kill and choke lines (8) in case of drift off, the EQD system comprising:

disconnectable hose connection units (50) for mud hoses (6) and kill and choke lines (8),

an extension line (53) connected to the disconnectable part of the mud hoses (6) and kill and choke lines (8) and the AHC system (31), wherein the extension line (53) can be paid out as the floating drilling unit (22) drifts off and the extension line (53) is long enough to lay the mud hose (6) and kill and choke lines (8) on the ocean floor before being cut or released.

a reel paying out power and control lines (9).

11. System for riserless drilling according to claims 9 or 10, wherein the system comprises a Dynamic Positioning (DP) system (33) for the floating drill ing unit (22).

12. System for riserless drilling according to claims 9 or 10, wherein the system comprises an anchoring system to keep the floating drilling unit in position.

13. System for riserless drilling according to any of the claims 9 to 12, wherein floating drilling unit (22) is a dual derrick drilling unit (24) comprising a first derrick (25) and a second derrick (26), wherein the first and second derrick (25, 26) is arranged for drilling interchangeably in the same wellhead (13).

14. Method for riserless drilling using the system for riserless drilling according to claim 13 comprising the steps of:

executing a first drilling operation with the first derrick (25) while preparing a second drilling operation with the second derrick (26), executing a second drilling operation with the second derrick (26) while preparing a third operation with the first derrick (25).

15. Method for riserless drilling using the system for riserless drilling according to claim 10 and 11 comprising the steps of:

executing a drilling operation,

if the floating drilling unit (22) exceeds a safe drilling region, activating the Emergency Quick Disconnect (EQD) system (32) for disconnecting the return mud hoses (6)and kill and choke lines (8) by disconnecting the disconnectable hose connection units (50) wherein the extension line (53) is paid out as the floating drilling unit (22) drifts off and lays the mud hose (6) and kill and choke lines (8) on the ocean floor before the extension line (53) is cut or released.